Typically, in the turbine section of gas turbine engines, turbine nozzles (also referred to as vane airfoils) are positioned forward of rotating buckets, and are utilized to direct hot combustion gases at an optimal angle to cause the buckets to efficiently rotate, which, in turn, produces power used to turn a shaft which, in the case of a gas turbine for power generation applications, may be connected to a generator for the production of electricity.
Gas turbine nozzles are typically hollow metal structures and are manufactured using the investment casting process. Current methods of investment casting of gas turbine nozzles include shaping the nozzle airfoil component in wax by enveloping a conventional alumina or silica based ceramic core which defines internal coolant passages of the nozzle. The wax assembly then undergoes a series of dips in liquid ceramic solution. The part is allowed to dry after each dip, forming a hard external shell, typically a conventional zirconia based ceramic shell. After all dips are complete, and the wax assembly is encased by several layers of hardened ceramic shell, the assembly is placed in a furnace where the wax in the shell is melted out. The remaining mold consists of the internal ceramic core, the external ceramic shell, and the space between the core and the shell, previously filled by the wax. The mold is again placed in the furnace, and liquid metal is poured into an opening at the top of the mold. The molten metal enters the space between the ceramic core and the ceramic shell, previously filled by the wax. After the metal is allowed to cool and solidify, the external shell is broken and removed, exposing the metal nozzle component which has taken the shape of the void created by removal of the wax, and which encases the internal ceramic core. This nozzle component is then placed in a leeching tank, where the ceramic core is dissolved. The metal nozzle component now has the shape of the wax form, and an internal cavity which was previously filled by the internal ceramic core.
The relative thermal growths of the ceramic shell and the ceramic core material are different, so that after the metal has been poured and is allowed to cool, the relative shrinking of the shell and core components are different. This can cause varying wall thicknesses at areas of the metal nozzle part where one side of the wall is defined by the external shell, and the other side of the wall is engaged by the internal core. In particular, and as explained in greater detail below, the region where the airfoil forms a fillet with the outer nozzle band has traditionally been a very difficult region in which to control casting wall thicknesses.
In FIG. 1, a typical turbine nozzle is shown at 10. The nozzle is comprised of an airfoil section 12, an outer nozzle band 14, an inner nozzle band 16, an inner mounting flange 18, an inner airfoil fillet 20A where the airfoil section 12 meets the inner nozzle band 16, an outer airfoil fillet 20B where the airfoil section 12 meets the outer nozzle band 14 (see FIG. 2), internal airfoil ribs 22, and an outer mounting hook 24. The turbine nozzle also has an outer vertically oriented collar 26B around the periphery of the airfoil section on the side of the outer nozzle band opposite the airfoil section 12 and at the interface between the fillet 20B and the outer nozzle band 14. A similar inner collar 26A is formed at the interface between the fillet 20A and the inner nozzle band 16.
With reference now also to FIG. 2, the nozzle 10 is shown with the internal alumina or silica based ceramic core 28 and the external zirconia based shell 30 as they would appear after pouring of the molten metal into the space previously described above. It should be pointed out here, however, that the nozzle as shown in FIG. 2 has the same shape as the temporary wax form and, therefore, surfaces or shapes of the temporary wax form correspond to identical shapes or surfaces of the metal nozzle. Accordingly, references herein to either the wax form or the resulting metal nozzle structure are, in effect, interchangeable. For example, the horizontally oriented ribs 26A and B are initially formed in wax and later formed by the molten metal poured into the space vacated by the wax. This is also true with respect to FIGS. 3 and 4 as described further herein.
Note that the core 28 has enlarged ends 32 (at the fixed end of the nozzle which is intended to be firmly attached in the turbine) and 34 (at the free end). At the "fixed end", there is little or no relative expansion between the ceramic core and shell. At the "free end", however, such relative expansion readily occurs. In the process of preheating the mold prior to metal pouring, and in cooling the mold after metal pouring, the external shell 30 and internal core 28 grow and shrink at different rates due to different material properties of the two ceramic materials. The wall thickness of the outer and inner bands 14 and 16, respectively, is not affected by this relative growth phenomena, since both sides of the metal bands are engaged by the same external shell material which, of course, has uniform thermal growth properties. Relatively consistent wall thickness in the areas of the inner and outer bands are therefore readily obtainable.
The thickness dimensions at the inner band wall fillet 20A is affected to only a minor, insignificant extent, since the shell and core are held to each other at this end, i.e., the "fixed end". There is thus a smaller distance over which the relative growth can occur, and as a result, the absolute relative growth is much smaller in comparison to the area opposite the fixed end.
In the region where the airfoil forms the fillet 20B with the nozzle outer band, i.e., at the "free end", however, the different growth and shrink rates readily occur, and it is here that differential thermal expansion significantly affects the wall thickness dimension. This region generally tends to be one of the areas of high stress and low part life, making it a critical region where wall thickness control is essential.